The conventional method of synthesizing substituted phenol ester involves the acylation in a solvent such as hexane or dichloroethane, of a salt of the substituted phenol with an acyl halide, (normally the chloride), eliminating hydrogen halide. This method, however, suffers from several disadvantages. The starting acyl halide is itself a difficult and unpleasant material to handle in bulk, and the equipment needed to withstand corrosive attack by the hydrogen halide byproduct of the reaction is expensive. A further problem arises in the elimination of the hydrogen halide from the reaction system. This is normally carried out by means of an inert gas sparging system but the tendency of the suspended solid reaction product to cause foaming of the reaction mixture, and the need to avoid removal of the solvent and/or the acyl halide, restricts the gas flow rate, and thus the completeness of removal. If HCl is not removed efficiently there is competition with the sulphonate group for sodium ions leading to sodium chloride and a less stable sulphonic acid form of the product.
An alternative synthesis route involves the preparation of the appropriate alkanoic anhydride, normally by means of a reaction between acetic anhydride and the appropriate carboxylic acid, followed by the further reaction of the alkanoic anhydride with the substituted phenol ester. In conventional practice, the two reaction stages are carried out separately. In the first stage the carboxylic acid and acetic anhydride, the latter being employed in excess and serving as a reaction medium, are refluxed from 6-8 hours and the unreacted acetic anhydride is then distilled off together with any acetic acid formed during the reaction. The alkanoic anhydride is then isolated by fractional distillation before being reacted with the substituted phenol ester.
The applicants have now found that the preparation of hydrophilic substituted phenol esters can be simplified by carrying out both reactions in a single reactor and by employing a catalyst, preferably of strong acid type, to reduce the severity of the reaction conditions.
According to the present invention there is provided a method of preparing a substituted phenol ester comprising the steps of:
(a) reacting a C.sub.2 -C.sub.3 alkanoic anhydride with a substituted or unsubstituted C.sub.6 -C.sub.18 aliphatic carboxylic acid in a molar ratio of anhydride:acid of at least 0.5:1; PA0 (b) volatilizing and removing the excess C.sub.2 -C.sub.3 alkanoic anhydride and any C.sub.2 -C.sub.3 carboxylic acid formed during the reaction, whilst maintaining the C.sub.6 -C.sub.18 acid anhydride in a fluid state; PA0 (c) reacting the C.sub.6 -C.sub.18 acid anhydride with a substituted phenol in the presence of a strong acid or base catalyst, the substituted phenol having the general formula ##STR1## wherein X is selected from (i) --COOH PA0 (d) recovering the substituted C.sub.6 -C.sub.18 acyloxy benzene from the reaction mixture.
(ii) --OSO.sub.3 M PA1 (iii) --SO.sub.3 M PA1 (iv) ##STR2## (v) --N.sup.+ R.sub.1 R.sub.2 R.sub.3 Y.sup.- wherein M is alkali metal or alkaline earth metal, each of R.sub.1, R.sub.2 and R.sub.3 is a C.sub.1 -C.sub.3 alkyl group, a is 0 or 1 and Y is a hydrophilic group selected from halide, methosulphate and ethosulphate radicals; and
In a preferred embodiment of the process, the substituted C.sub.6 -C.sub.18 acyloxybenzene is a C.sub.6 -C.sub.10 acyloxybenzene sulphonate and the catalyst is a strong acid catalyst such as H.sub.2 SO.sub.4. Preferably the C.sub.6 -C.sub.18 aliphatic carboxylic acid byproduct from the second stage of the reaction is recovered and recycled to the first stage of the reaction.